The typical contact-type reading apparatus features a series of standard elements that are herein detailed. First of all, the apparatus has a contact glass for keeping the surface of the document at a constant distance. A linear array of light-emitting diodes (LEDs) is used for illuminating the portion of the document to be read. Said LED array sends rays of light through a light guide, which purpose is to focus the light onto the surface of the document to be read. Usually the LED array and the light guide span the entire reading length of the apparatus. The incident rays of light are reflected by the illuminated portion of the document surface, and the reflected rays are focused by an array of optic fiber lenses onto an array of optical sensors. This optical sensor array comprises one or more solid state devices deploying multiple individual photo cells in a linear array. Said optical sensor converts the image focused onto it in electrical signals that are digitally codified, resulting in a digital image that can be stored in an internal memory for future use.
This same concept does not lend itself to application in smaller scales, namely for portable contact-type reading apparatuses. Properly even illumination of the document surface is critical to the performance of the device, and so an array of LEDs spanning the entire length of the reading apparatus is normally required. Such an array would entail a larger size incompatible with the desired small dimensions of a portable device. Furthermore, the amount of energy required to simultaneously power the range of LEDs would require sizeable battery storage capacity, again yielding incompatibility with a small sized device.
The prior art includes U.S. Pat. No. 6,441,928 by the same Applicant, which discloses a different approach wherein the linear array of LEDs is replaced by a light pipe. Said light pipe comprises a clear pipe made of glass or plastic, with a single LED deployed at each end. Considering as reference a plan containing the central longitudinal axis of the light pipe, the section above said plan is termed the back of the light pipe, while the other section is termed the front of the light pipe, where the contact glass is located and where the paper surface is positioned for reading. The use of only two LEDs overcome the power requirement problem, but introduces the need to properly direct the LEDs light rays towards the document surface. The inside back surface of the pipe is painted white, forming a reflective surface. As the LEDs discharge light rays through the pipe, part of said light rays is reflected by the white paint, causing the light to scatter and disperse evenly, therefore illuminating the document as it passes over the contact-glass. Then, the illuminated portion of the document is reflected by the surface of the document, passes through a waveguide and is focused by an optical lens array onto an optical sensor array integrated in the portable contact-type reading apparatus. The optical sensor array converts the image received into electrical signals to produce a digital image to be stored in an internal memory.
Although it addresses the basic problem of making the contact-type reading apparatus compact and reducing electric power demand, the device disclosed in U.S. Pat. No. 6,441,928 still has some inconveniences. The illumination potential of the LEDs is not fully tapped, and the resulting image resolution—critical for the device performance—falls short of what would be afforded by better illumination.